Natural gas is a combustible, gaseous mixture of several different hydrocarbon compounds and is typically extracted from deep underground reservoirs formed by porous rock. The composition of natural gas extracted from different reservoirs varies depending on the geographic location of the reservoir. In fact, it is not entirely uncommon for the composition of gas extracted from a single given reservoir to vary to an extent. Regardless of any variations, however, the primary component of natural gas is methane, a colorless, odorless, gaseous saturated hydrocarbon. Methane usually accounts for 80% to 95% of any natural gas sample and the balance is composed of varying amounts of ethane, propane, butane, pentane and other hydrocarbon compounds.
Natural gas is used extensively in residential, commercial and industrial applications. It is the dominant energy used for home heating with well over half of American homes using natural gas. The use of natural gas is also rapidly increasing in electric power generation and cooling, and as a transportation fuel.
Natural gas, like other forms of heat energy, is measured in British thermal units or Btu. One Btu is equivalent to the heat needed to raise the temperature of one pound of water by one degree Fahrenheit at atmospheric pressure.
A cubic foot of natural gas has about 1,027 BTU. Natural gas is normally sold from the wellhead, i.e., the point at which the gas is extracted from the earth, to purchasers in standard volume measurements of thousands of cubic feet (Mcf). However, consumer bills are usually measured in heat content or therms. One therm is a unit of heating equal to 100,000 BTU.
Three separate and often independent segments of the natural gas industry are involved in delivering natural gas from the wellhead to the consumer. Production companies explore, drill and extract natural gas from the ground; transmission companies operate the pipelines that connect the gas fields to major consuming areas; and distribution companies are the local utilities that deliver natural gas to the customer.
In the United States alone, natural gas is delivered to close to 200 million consumers through a network of underground pipes that extends over a million miles. To produce and deliver this natural gas there are over a quarter-million producing natural gas wells, over one hundred natural gas pipeline companies and more than a thousand local distribution companies (LDCs) that provide gas service to all 50 states.
Prior to regulatory reform, which essentially restructured the industry, producers sold gas to the pipeline companies, who sold it to the LDCs, who sold it to residential and other customers. Post-regulation, however, pipeline companies no longer purchase gas for resale. Instead, the pipeline companies merely transport gas from sellers, such as producers or marketers, to buyers, such as electric utilities, factories and LDCs. Thus, the LDCs now can choose among a variety of sellers of natural gas, whereas before they could only buy gas from one source, i.e., the pipeline company. Further, some states have implemented additional restructuring which renders the LDCs subject to regulation by State public utility commissions. Prior to these additional state regulations, an LDC's residential customers could only buy gas from one source, i.e., the LDC. After state regulation, however, residential customers can choose a different supplier other than their LDC from which to buy the gas. The consumer's LDC, as the owner/operator of the distribution network, delivers the gas to the consumer, as before, but the LDC only charges the consumer for delivery of the gas and the independent supplier bills for the gas.
Thus, natural gas is very important to the U.S. energy supply. Consumption of natural gas in the United States, however, has increased beyond the available supply of domestic natural gas. One available option to increase supply is to increase imports of liquefied natural gas (LNG).
More particularly, according to one estimate natural gas consumption in the United States is expected to increase from about 22 trillion cubic feet (Tcf) in 2004 to almost 31 Tcf by 2025. Accordingly, domestic production combined with imports via pipeline from Canada will be insufficient to meet the demand. In response, a small but growing percentage of gas supplies are imported and received as LNG via tanker ships.
LNG is produced by taking natural gas from a production field, removing impurities, and liquefying the natural gas. In the liquefaction process, the gas is cooled to a temperature of approximately −260 degrees F. One volume of this condensed liquid form of natural gas occupies about 1/600th of the volume of natural gas at a stove burner tip. The LNG is loaded onto double-hulled ships which are used for both safety and insulating purposes. Once the ship arrives at the receiving port, the LNG is typically off-loaded into well-insulated storage tanks. Vaporization or regasification is used to convert the LNG back into its gas form, which enters the domestic pipeline distribution system and is ultimately delivered to the end-user.
Because LNG is sold in accordance with its BTU value, accurate analysis of the BTU value of any particular LNG shipment, as well as analysis of the constituent components of the LNG, as it is off-loaded from a respective tanker ship is crucial. For example, to determine an expected price for a particular shipment, when LNG is loaded onto a tanker ship at an overseas location, such as Trinidad and Tobago where large natural gas reserves are found, the supplier calculates the Btu value of the LNG as it is loaded into the hull of the ship. Additionally, because the Btu value of the shipment will likely change in transit, for example due to vaporization of some of the LNG while it is sitting in the hull of the ship, the recipient of the LNG shipment also desires to accurately determine the Btu value of the delivered LNG shipment. The operator of the tanker ship carrying the LNG shipment is also keenly interested in accurate BTU measurement of both the loaded LNG as well as the off-loaded LNG as the shipper typically burns the LNG vaporized in transit to run the ship and, thus, is responsible for cost of the LNG vaporized in transit.
Accordingly, it is desired to provide a method and system for accurately measuring the BTU value of an LNG shipment as it is off-loaded from a tanker ship.
One related art method that addresses the issue discussed above is disclosed in U.S. Pat. No. 3,933,030 to Forster et al. In Forster, a system is disclosed for the continuous monitoring of the density of cryogenic liquids, such as LNG. In accordance with the Forster system the dielectric constant of stored LNG is instantaneously determined by the use of sensors in the storage tank. Multiple sensors, each comprising a capacitor probe, are placed at various locations within the storage tank. The sensors are then operable to instantaneously measure the dielectric constant of the liquid within the tank and from this data the density of the liquid in the tank is determined. From the density measurement it is possible to then calculate the BTU per unit volume and appropriate charges per BTU can be calculated.
Several problems arise from a system such as the one disclosed in Forster, however. For example, the accuracy of the BTU measurement is unacceptable for today's standards.
Other, more recent, related art systems utilize chromatograph technology to determine the BTU value of LNG. These related art systems, however, also suffer from poor accuracy and/or high levels of maintenance. For example, one known system utilizes a method in which liquid gas is circulated in tubes that are submersed in a heated solution. The heat in the solution, in turn, heats the tubing which vaporizes the liquid gas. This method of vaporization is very inefficient, however, and the accuracy of any resulting BTU measurements are unacceptable, e.g., less than 5 BTU, that is, the swing on the BTU measurement is greater than 5 BTU.
Accordingly, it is desired to provide a system that does not suffer from at least these problems and which can provide a much more accurate and detailed assessment of liquefied gas and at the same time requires less maintenance than current systems.